@yannickq Sure, it's not correct to write that, but I can guarantee you that I have said something like that at least more than once. It's the kind of thing that can happen in actual speech, an awkward pause with a sudden correction or added thing tacked on. Beats the hell out of starting an entire sentence over from scratch to fix one little thing. To me, this reads like him stumbling over that part because he's got
decades worth of a habit in referring to her as his master, and about two seconds worth of referring to her as this sixth disciple that he just pulled out of thin air. There might be something to the suspense angle too, but an awkward pause feels like a better interpretation to me. Certainly might not be what the author was going for, but I like it.